1. Field of the Invention
The process of the invention relates to the conversion of sulfur dioxide to sulfur trioxide, and derivatives of sulfur trioxide, including sulfuric acid. The process is useful for removing sulfur dioxide from stack gas, such as exhaust and waste gas effluents associated with various industrial processes, such as: (1) chemical processes; (2) combustion of coal and oil; and (3) processes for treating liquid and solid wastes.
2. Description of the Prior Art
The presence of sulfur dioxide in the atmosphere has become a matter of great importance in recent times. Sulfur dioxide is now a serious contaminant in the atmosphere, and can adversely affect both living and inanimate objects which are exposed to it, both as sulfur dioxide per se, and as sulfuric acid, to which sulfur dioxide may ultimately be converted under certain atmospheric conditions. See, e.g., Bufalini, Environmental Science and Technology, Vol. 5, pages 685-699 (1971); Sidebottom et al, Environmental Science and Technology, Vol. 6, pages 72-79 (1972); and Bricard and Vigla, Canadian Journal of Chemistry, Vol. 52, pages 1485-1490 (1974). Although it is well known that sulfur dioxide can be oxidized in the atmosphere, the intermediate species involved and the reaction paths remain open to question.
Since sulfur dioxide is produced as a gaseous by-product in many chemical reactions which are useful to modern society, the removal of sulfur dioxide from gaseous effluents has become the subject of intensive investigations. The chemical reactions which produce the sulfur dioxide are many in number, including: (1) industrial processes; (2) the process of combustion of coal and oil containing sulfur compounds as minor constituents, such as by electric power generating plants; and (3) various processes for treating liquid and solid wastes. Most often the sulfur dioxide is just a minor constituent of the gaseous effluent from the process. Such gaseous effluents are commonly referred to as stack or flue gas.
Many processes have been devised in the attempt to economically and efficiently remove sulfur dioxide from stack gas. The most widely used method, involving wet limestone scrubbing, generates large amounts of a waste sludge including gypsum, calcium sulfite, and fly ash. Desirably, the process for removing sulfur dioxide should produce useful by-products without producing other waste materials.
Other processes for removing sulfur dioxide from stack gases, and for converting sulfur dioxide to other substances, involving reactions with various other intermediates and catalysts, are known. Thus U.S. Pat. No. 3,251,649, issued May 17, 1976, describes a process in which a waste gas containing sulfur oxides is contacted with a water suspension of manganese oxide to form manganese sulfate, which is then treated with hydrogen chloride to form sulfuric acid. Through a series of subsequent reactions, the manganese oxide is ultimately regenerated. U.S. Pat. No. 3,615,196, issued Oct. 26, 1971, describes removal of sulfur dioxide from flue gas by means of vanadium pentoxide catalyst absorbent; the process is carried out at high temperature, e.g., 1,000.degree. C. See also U.S. Pat. Nos. 3,987,153, issued Oct. 19, 1976, and 4,012,487, issued Mar. 15, 1977, which describe methods for removing sulfur dioxide from exhaust gases, involving complex reactions with special catalysts and intermediates.
Processes have been devised for conversion of sulfur dioxide using oxygen in the form of ozone, as described in South African Pat. No. 69 07638, May 27, 1970, or by a combination conversion technique employing both ozone and manganese oxide catalyst, as described in U.S. Pat. No. 3,574,562, issued Apr. 13, 1971. Although these patents indicate that oxygen can react directly with sulfur dioxide while in the highly reactive form of ozone, ozone oxidation of sulfur dioxide is not favored economically because of the relatively high energy input and low ozone output of ozone generating equipment. Although reactive oxygen in the form of ozone (triatomic oxygen) can be prepared by other techniques, these techniques have drawbacks which hinder their utilization for the removal of sulfur dioxide from stack gas.
Oxygen normally exists as a diatomic molecule in the triplet electronic state. Diatomic oxygen is also known to exist in an electronically excited singlet state. Singlet oxygen can be generated by photolytic techniques involving use of oxygen-photosensitizing dyes. Known physical and chemical properties of singlet oxygen are described, for instance, by Kearns, D. R., Chemical Reviews, Vol. 71, pages 395-427 (1971). Singlet oxygen is known to react with certain diene, aromatic, and olefin compounds. U.S. Pat. No. 4,008,136, issued Feb. 15, 1977, describes singlet oxygen generated using an oxygen-photosensitizing dye attached to a polymer support, and its reaction with: (1) 2-methyl-2-butene to form 3-methylbut-3-ene-2-ol and 2-methylbut-3-ene-2-ol; (2) anthraceneto form anthracene endoperoxide; (3) 2,3-diphenyl-p-dioxene to form 1,2-ethanediol dibenzoate; and (4) sewage water, and the reduction in the coliform count resulting therefrom. The reactivity of singlet oxygen with other substances is largely unknown.